Custom Column, add value from unrelated table based on date range

Use Table.Addcolumn with a custom ColumnGenerator function that uses your logic to return rows from the second table that satisfy your condition, and then pick the required column (Label).

Here is an example for the Table 1 query based on your definition for Table 2:

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMtQ31TcyMDJQitUBcQwtEDwjfRgnFgA=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Date = _t]),
    #"Added Custom" = Table.AddColumn(Source, "Lookup", (k)=> 
    Table.SelectRows(#"Table 2",
      each (Date.From(k[Date])>= Date.From([Start Date]) and Date.From(k[Date])<= Date.From([End Date])))
    ),
    #"Expanded Lookup" = Table.ExpandTableColumn(#"Added Custom", "Lookup", {"Label"}, {"Label"})
in
    #"Expanded Lookup"